To reduce harmful emissions, combustion processes which are spark-ignited but which are then self-running are frequently operated in gas turbines by a main-flow fuel system or a main injection with a lean gas-air mixture generated outside the respective flame chamber. However, the lean gas-air mixture results in a narrow stability range in the respective combustion process. The narrower the stability range, the more susceptible the combustion process to changing operating conditions, which can result in an unintentional termination of the combustion process. For purposes of stabilizing the combustion process, known premix burners have a pilot gas system by which a pilot flame is formed in the flame chamber from a fuel that is not premixed. Apart from stabilizing the combustion process, the pilot flame often also ignites the respective combustion process. Consequently, it is positioned in the vicinity of an ignition orifice of the premix burner. In so doing, stabilization of the combustion process is given priority over harmful emissions.
A premix burner for gaseous fuels and a method for controlling this premix burner are shown in DE 10 2005 054 442 A1. This premix burner has a multi-stage main injector with a plurality of main nozzles for injecting a gas-air mixture into a flame chamber and a central pilot gas system. In order to ignite the gas-air mixture, a pilot flame is formed by the pilot gas system from unmixed main fuel. Only a defined quantity of main nozzles are opened from the start until a predetermined load ratio. When falling below or exceeding a predetermined load ratio, additional main nozzles are switched on or main nozzles which are already operating are increased, respectively.
Another premix burner for gas turbines and a method for operating a premix burner of this kind are described in DE 103 34 228 A1. This premix burner for gaseous fuels has a main injector and a central multi-stage pilot gas system. The main injector has a group of main nozzles for injecting a premixed main fuel. The central multi-stage pilot gas system has a group of diffusion nozzles for injecting unmixed diffusion fuel and an independently controllable group of premix nozzles for additional injection of premixed main fuel.
When the premix burner is ignited, the greater portion of fuel is introduced by the diffusion stages of the pilot gas system. As the load increases, the premix nozzles of the pilot gas system and the main nozzles of the main injector are switched on. The diffusion nozzles are reduced. At full load, the premix nozzles and main nozzles are fully operated and the diffusion nozzles are reduced to a minimum of the total mass flow of fuel.
This method has the disadvantage that the stability range of the combustion process is not substantially enlarged in spite of the multi-stage pilot gas system. Further, a complicated control of the fuel supply is necessary and the premix burner reacts sensitively to changing fuel compositions.